Contact-free photomixing probe for device and integrated circuit measurement or characterization

ABSTRACT

A device for measuring and characterizing solid-state devices or integrated circuits at RF frequencies up to 1.0 THz and beyond is provided that includes a transmitting photomixing probe structure and a receiving photomixing probe structure. The transmitting photomixing probe structure and the receiving photomixing probe structure are ac-coupled to the solid-state device or integrated circuit in a contact-free manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of International Application No.PCT/2013/023633, filed Jan. 29, 2013 which claims the benefit of U.S.Provisional Patent Application Ser. No. 61/592,295, entitled “THzPHOTOMIXING PROBE FOR CONTACT-FREE DEVICE AND INTEGRATED CIRCUITMEASUREMENT OR CHARACTERIZATION,” filed Jan. 30, 2012. The entirety ofthe above-noted applications are incorporated by reference herein.

NOTICE ON GOVERNMENT FUNDING

This innovation was made with government support under a grant awardedby the Office of Naval Research (ONR) Award N00014-11-1-0721. Thegovernment has certain rights in the innovation.

ORIGIN

The innovation disclosed herein relates to contact-free measurement orcharacterization of solid-state devices or integrated circuits at RF andTHz frequencies, and more specifically, to a contact-free, ac-coupledphotomixing probe for use in such measurement or characterization.

BACKGROUND

One challenge to the development of THz semiconductor devices,particularly transistors, is THz measurement and characterization. As itstands today, vector network analysis (VNA) with standard(metal-to-metal) dc-coupled contact probing is commercially available upto 500 GHz (Oleson Microwave,http://www.omlinc.com/products/vna-extension-modules/wr-022-325-500-ghz.html)using coaxial probes (GGB Industries, Inc.; PicoProbe Model 325;http://www.ggb.com/325.html), with promise of extending to 750 GHz usingsilicon-micromachined probes (Dominion MicroProbes, Inc; Model DMPI1.5V01MPR; http://dmprobes.com/Products%20DMPI.html). However, thistechnology is very expensive and fragile, with little hope of workingbeyond 1.0 THz in the foreseeable future, in large part because itrequires application of vector network analyzers and custom front-endfrequency-extension modules and down-conversion units. The fragilitystems from the small size of the ground-signal-ground (GSG) probes andtheir need to make intimate metal contact. In technical terms, theexisting technology couples the conduction current term, J, in Maxwell'sequations (specifically, Maxwell's generalization of Amperes Law(∇×H=J+∂D/∂t)). In circuit terminology, this is “dc coupling.”

Photomixing entails the use of two fiber-coupled, frequency-offset diodelasers (usually distributed feedback lasers) to generate or receive asingle difference-frequency tone in a photomixer—an ultrafastphotoconductive gap easily embedded in a terahertz (THz) antenna orplanar transmission line. The tone is “pure” in the sense that there areno harmonics and no intermodulation products of any sort. An importantdevelopment over the past 10 years is fully-coherent photomixing. Thereis a transmit (Tx) photomixer and a receive (Rx) photomixer, both drivenby the same pair of diode lasers so that the difference frequency toneat each photomixer is mutually coherent. Difference-frequency sweepingoccurs by temperature tuning of one or both lasers. Mixing of theTx-radiated tone with the Rx-generated tone produces a dc component thatis easily read out to a transimpedance amplifier. This is the same“homodyne” conversion common in RF and photonic transceivers. To avoiddc-drift and 1/f-noise issues, the Tx photomixer is easily amplitudemodulation (AM) or frequency modulation (FM) modulated, and thephase-preserving Rx baseband is raised to the modulation frequency(quasi-homodyne).

SUMMARY

The following is a simplified summary to provide a basic understandingof some aspects of the innovation. This summary is not an extensiveoverview of the innovation. It is not intended to identify key/criticalelements or to delineate the scope of the innovation. Its sole purposeis to present some concepts of the innovation in a simplified form as aprelude to the more detailed description that is presented later.

In accordance with aspects of the innovation, a method for measuringsolid-state devices at frequencies up to 1.0 THz and beyond is providedthat includes a transmitting photomixing probe structure and a receivingphotomixing probe structure. The transmitting photomixing probestructure and the receiving photomixing probe structure are ac-coupledto the solid-state-device circuit in a contact-free manner.

In accordance with another aspect of the innovation, the method utilizescontact-free, ac-coupled probes that are fabricated from the samehigh-resistivity (e.g., silicon (Si), gallium arsenide (GaAs), indiumphosphide (InP), gallium nitride (GaN) etc.) substrates as thephotomixers and have either a fork-like or orthorhombic shape forprobing balanced THz transmission lines (e.g., coplanar waveguide(CPW)). The ac coupling occurs through polarization current, which isnaturally strong in Si, GaAs, InP, and GaN because of their highdielectric constant (∈≧12) and low THz dielectric loss. And unlikemetal-to-metal coupling that becomes increasingly lossy with frequency,dielectric coupling improves approximately linearly with frequency inaccordance with Maxwell's equation (Ampere's generalized equation).

The dielectric-coupling probe concept was inspired in part by“electro-optic sampling” first demonstrated in the 1980s and applied toa variety of high-frequency devices and structures in the 1990s.Electro-optic sampling is a time-domain technique that takes advantageof the short pulses (˜100 fs) and very stable repetition frequency(typically 10 to 100 MHz) of modern solid-state mode-locked lasers(e.g., titanium sapphire). The optical pulses create short THz pulses(typically<1 ps), which then interact with the device or circuit undertest, and produce an output that is essentially the impulse response. Solike all time-domain techniques, the THz power is spread over 1 THz ormore, which degrades the instantaneous signal-to-noise ratioconsiderably compared to a coherent transceiver. This can be overcome byaveraging but then the data acquisition time becomes quite long, on theorder of many minutes, which is usually unacceptable for devicecharacterization.

To create the foregoing and related capabilities, certain illustrativeaspects of the innovation are described herein in connection with thefollowing description and the annexed drawings. These aspects areindicative, however, of but a few of the various ways in which theprinciples of the innovation can be employed and the subject innovationis intended to include all such aspects and their equivalents. Otheradvantages and novel features of the innovation will become apparentfrom the following detailed description of the innovation whenconsidered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of THz ac-coupled probe structures coupledto a device in accordance with aspects of the innovation.

FIG. 2 is an illustrative graph of a typical output signal, noise floor,and signal-to-noise ratio from a coherent photomixing spectrometer.

FIG. 3 is a plan view of a proposed balanced “microfork” THz probestructure in accordance with aspects of the innovation.

FIG. 4 is a plan view of an unbalanced “microfork” probe structure.

FIG. 5 is an illustration of micrograph views of laser micromachiningfabricating techniques in accordance with aspects of the innovation.

FIGS. 6-9 are illustrations of fabrication techniques in accordance withaspects of the innovation.

FIG. 10 displays the physical methodology behind the proposed techniqueof measuring or characterizing a solid-state device or integratedcircuit with contact-free probes at frequencies up to 1.0 THz and beyondin accordance with aspects of the innovation.

FIGS. 11A-B show a plan view of a proposed “orthorhombic” THz probestructure in accordance with aspects of the innovation, and designedwith coplanar-waveguide coupling for measuring the signals in similarbalanced circuits such as that shown in FIG. 11B.

FIGS. 12A-B show a plan view of a proposed “orthorhombic” THz probestructure in accordance with aspects of the innovation, and designedwith slot-line coupling for measuring signals in similar unbalancedcircuits such as that shown in FIG. 12B.

DETAILED DESCRIPTION

The innovation is now described with reference to the drawings, whereinlike reference numerals are used to refer to like elements throughout.In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the subject innovation. It may be evident, however,that the innovation can be practiced without these specific details. Inother instances, well-known structures and devices are shown in blockdiagram form in order to facilitate describing the innovation.

While specific characteristics are described herein (e.g., specificdimensions of example components, etc.), it is to be understood that thefeatures, functions and benefits of the innovation can employcharacteristics that vary from those described herein. Thesealternatives are to be included within the scope of the innovation andclaims appended hereto.

While, for purposes of simplicity of explanation, the one or moremethodologies shown herein, e.g., in the form of a flow chart, are shownand described as a series of functions, it is to be understood andappreciated that the subject innovation is not limited by the order offunctions, as some may, in accordance with the innovation, occur in adifferent order and/or concurrently with other functions from that shownand described herein. For example, those skilled in the art willunderstand and appreciate that a methodology could alternatively berepresented as a series of interrelated states or events, such as in astate-flow diagram. Moreover, not all illustrated functions may berequired to implement a methodology in accordance with the innovation.

With reference now to the figures, the innovation disclosed hereinprovides a contact-free system and method of creating an input signal,and probing the output signal of a solid-state device well beyond thefrequency limits of existing contact probing. The subject innovationenables operation at higher frequencies and with greater instantaneousbandwidth than currently possible. The approach is to probe the deviceoutput by way of the displacement current term (∂D/∂t), referred to as“ac coupling” in circuit terminology. Although ac coupling is lower inefficiency than dc coupling at microwave frequencies, it becomesincreasingly more efficient as the frequency increases. It also improveswith the dielectric constant of the coupling material. Accordingly, theinnovation disclosed herein can provide a new device measurement andcharacterization capability at frequencies up to 1.0 THz and beyond thecapabilities of existing probing and vector network analysis (VNA)techniques. In addition, the subject innovation allows one to measureand characterize the device without the probes coming into electricalcontact with the planar transmission line in which the sample isembedded. As such, the subject innovation avoids the mechanical stresson the probe and scratch-induced damage (of the sample transmissionline) that existing contact probe technology entails.

Referring to FIG. 1, illustrated is a perspective view of a solid-statedevice 102, such as but not limited to a high-speed THz Si-, GaAs-,InP-, or GaN-based semiconductor device, to be tested while embedded ina planar transmission line 104 with the innovative contact-free,ac-coupled probes. In some examples discussed herein, specific examplesof a planar transmission line 104 are provided (e.g., a coplanarwaveguide (CPW), etc.), but it is to be appreciated that other coplanartransmission lines may be used, such as slot line, microstrip, and twinline. The ac-coupled probes include a transmitting photomixing probestructure, Tx 106, and a receiving photomixing probe structure, Rx 108.Each of the probes 106 and 108 include a photomixer 106A, 108A at thephotoconductive gap of the probes. Optical fibers 110, 112 couple thephotomixers 106A, 108A, respectively, to two external lasers (notshown). The two lasers need to be mutually coherent, but their frequencydifference needs to be tunable. In an aspect of the innovation, both thelasers are single frequency diode lasers. In addition, each of theprobes 106 and 108 includes a planar transmission line structure 106C,108C at the photomixer 106A, 108C end of the probes, where the planartransmission line structure 106C, 108C includes a metalized surface106E, 108E. Each of the probes 106, 108 is made of a substrate 106D,108D, and a metalized surface 106E, 108E. In aspects, the substrate106D, 108D can be made of high resistivity, semiconducting material,including but not limited to, silicon (Si), gallium arsenide (GaAs),indium phosphide (InP), or gallium nitride (GaN). The probes 106 and 108can have many different designs and some but not all of them will bediscussed in connection with FIGS. 3, 4, 11A-B, and 12A-B. In someembodiments of the innovation, such as the examples shown in FIGS. 3 and4, each of the probes 106 and 108 can also include a dielectricwaveguide structure 106B, 108B (without any metalized surfaces) at thecircuit end of the probes, where the circuit end of the probes is theend closer to the device or circuit under test. The thickness of theprobes 106, 108 can be in the range of 10 to 1000 micron, for example,around 100 micron. The height is in the range of 0.1 to 10 mm, forexample, around 1.0 mm. In various aspects, probes 106, 108 can be madeusing standard semiconductor fabrication and packaging methods,especially if they are made with a silicon substrate.

Coupling to the solid-state device 102 occurs through the dielectrictransmitting and receiving photomixing probes 106, 108 acting in partlike near-field, dielectric-rod or dielectric-waveguide antennas. Theoutput field of the solid-state device 102 can be sampled in thecoplanar-waveguide or other planar transmission line 104 in which thesolid-state device 102 is embedded. The gap distances D₁ and D₂ from theprobes 106 and 108 to the planar transmission line 104 is a trade-offbetween weak coupling (at large D) and strong perturbation of thecircuit 102 (at small D). While D₁ and D₂ do not have to be the same,they can fall in the range between 1 and 10 micron, for example, around10 micron. The coupling in this range is “near-field,” meaning that theelectromagnetic mode excited in the dielectric waveguide structure 106Band 108B of the probes propagates through polarization currents∂P/∂T=χ∂E/∂T, where χ is the electric susceptibility given by χ=∈−1,where ∈ is the THz dielectric constant. The polarization current is, inturn, transformed to conduction current by a metallic transformingtransmission line (106C and 108C) located well above the dielectricprongs. An example transforming structure (106C and 108C) can be aterminated section of coplanar waveguide (CPW) or other planartransmission-line structure. Tapering of the transmission line may benecessary to achieve acceptable efficiency, and may be achieved by avariety of methods including “adiabatic” transformation as is done inthe well-known Klopfenstein coupler at microwave frequencies. As such,because D₁ and D₂ are not equal to zero, the subject innovation canenable the probes 106, 108 to measure or characterize the device/circuitunder test 102 without the probes 106, 108 coming into electricalcontact with the planar transmission line 104 in which the device orcircuit under test 102 is embedded. This contact-free capability givesthe subject innovation the ability to avoid the mechanical stress on theprobe 106, 108 and scratch-induced damage (of the sample transmissionline 104) that the prior art experiences.

Once the polarization current is transformed to conduction current, thefield strength can be measured by an optical fiber-coupled photomixer108D monolithically embedded into the CPW or other transmission line106C, 108C at the top of the receiving photomixing probe 108. THzphotomixing is well-suited to this task because of its huge operationalbandwidth, its spectral purity, and lack of harmonics, spurs, and othernon-linear effects that occur when scaling the conventionalvector-network analyzer into the THz regime. Photomixing is so purebecause the THz tone it generates is exactly the beat frequency |v₁−v₂|between two frequency-offset, single-frequency diode lasers, readilyavailable in either DFB (distributed feedback) or DBR (distributed Braggreflector) designs. There are no harmonics in this process, nor spurs,because the nonlinearity that the mixing action is based upon is purequadratic (the fundamental internal photoelectric effect associated withcross-gap absorption in any semiconductor). Assuming it is sinusoidal atfrequency f, the incident signal across the transmission line in thereceive probe is then down-converted to an intermediate frequency (IF)given by |f−|v₁−v₂∥.

A notable aspect of the subject innovation is that the THz coupling ofthe contact-free probe is reciprocal. That is, the coupling to atransmission-line under test 104, and the subsequent transformation to atransmission-line-embedded photomixer, can also operate in reverse.Hence, a THz signal generated by the photomixer 106A will propagate inthe probe planar transmission line 106C, transform to adielectric-waveguide mode in dielectric waveguide structure 106B, andcouple to the device or circuit under test 102 through near-field,polarization-current coupling without electrical contact with planartransmission line 104. The signal from the device or circuit under test102 will couple to the dielectric waveguide structure 108B withoutelectrical contact with planar transmission line 104, transform andpropagate to the probe planar transmission line 108C before reachingphotomixer 108A.

The operation of the transmit and receive photomixers is based on thesame laser-generated THz difference-frequency tone. However, thetransmit photomixer 106A must generate a THz signal while the receivephotomixer 108A only needs to accept it. This difference can be readilycontrolled by DC bias. The transmit photomixer 106A can be DC-biased sothat the laser-difference-frequency tone transforms DC bias power to ACTHz power through photoconductance modulation. The receive photomixer108A need not be DC biased, in which case an incident THz signal willmix with the laser-difference-frequency tone. Lacking any modulation ofthe transmit laser signals, this is pure homodyne conversion. To address1/f and related problems, one of the transmit laser powers can be AMmodulated to produce an IF well within the bandwidth of lock-inamplifiers and ultra-low-noise transimpedance amplifiers. This“quasi-homodyne” approach carries the fully advantages of traditionalhomodyne and heterodyne conversion typically used in vector networkanalyzers and coherent RF transceivers of all sorts. One significantadvantage over incoherent detection (which spectrum analyzers usuallydo) is signal-to-noise ratio. Homodyne and heterodyne transceivers canroutinely operate near the theoretical limits dictated by electronicnoise. This provides a very high dynamic range, usually in the range of50-to-100 dB depending on the frequency band and the electronicsinvolved.

The transmitting photomixer probe 106 and receiving photomixer probe 108can be driven by the same pair of frequency-offset lasers, so thetypical frequency jitter in the diode lasers is cancelled perfectly,meaning that there is an inherent capability to do frequency or phaselocking. This assumes, of course, that the jitter is tolerably smallcompared to the THz frequencies of interest. Modern DBR and DFB lasersboth tend to jitter by about 100 MHz or less, or 1 part in 10⁴ of a THztone, and there is no anticipation of resolution requirements any finerthan this in the course of the typical THz device characterization.

FIG. 2 illustrates the spectral response, quadrature-componentamplitude, and dynamic range of a commercial quasi-homodyne photomixingtransceiver (model PB7100 manufactured by Emcore, Corp.). The dynamicrange is seen to be 80 dB at 100 GHz, 60 dB at 1.1 THz, and still 40 dBat 2.0 THz. Although the THz radiation propagates as quasi-Gaussianbeams rather than the evanescent/surface or dielectric-waveguide modesdisclosed herein, the operating principle will be the same, andcomparable values of dynamic range are expected once the dielectricprobe designs are optimized.

FIG. 3 illustrates a plan view of an example contact-free, ac-coupledprobe structure 106 or 108 (hereinafter “probe”) in a balanced microforkdesign configuration that is compatible with a planar transmission line104 that is a CPW in accordance with aspects of the innovation (FIG. 4is an illustration of a probe structure 400 in an unbalanced microforkdesign that is compatible with a planar transmission line 104 that is amicrostrip). The probe 300 includes dielectric prongs/dielectricwaveguide structure 302, a tapered region/planar transmission linestructure 304 and an interdigital electrode region 306. The probe can bemade from a high-resistivity Si, GaAs, InP, or GaN substrate materialbetween, 10 and 1000 microns thick, for example, around 100 micronsthick. The overall height of the probe 300 can be between 0.1 and 10 mm,for example, around 1.0 mm high. The dielectric prongs 302 can bebetween 0.05 and 5 mm high. Further, the bottom portion of eachdielectric prong 302 can be between 10 and 100 microns wide.

The approximate dimensions illustrated in FIGS. 3 and 4 are based onpreliminary design considerations, including the width of theCPW-transmission-line 104 circuit in which the device-under-test 102 islikely to be embedded, the necessary offset of the dielectric-waveguideportion of the probe to the transmission-line circuit, and the areaneeded for fiber-optic coupling to the photomixer element. It is to beunderstood, of course, that the specific dimensions provided inconnection with example probe 300 are meant to illustrate one possibleembodiment, and greater or lesser values can be used in otherembodiments, such as where some of these design considerations vary. Thenon-rectilinear geometry of the “microfork” does not comply withstandard dicing or scribe-and-break tools. And because the thicknesswants to be approximately 100 micron or more for mechanical strength,wet- or reactive-ion etching of the substrate, be it Si, GaAs, InP, orGaN, would be cumbersome and expensive. Thus, an alternative approach todicing or scribing of the chips is laser micromachining—a technique thatworks very well on all common semiconductors and produces small kerf (10s of microns) and nearly vertical smooth sidewalls, see FIG. 5. This canbe accomplished via high peak-power Q-switched solid-state (e.g.,Nd:YAG) lasers that have become very popular in microelectronicfabrication the past decade. The dicing action occurs by laser ablation,which can penetrate up to several hundred microns over a short period oftime (minutes). And complex chip shapes and sizes can be obtained byrouting the laser beam with minors mounted on precision translationstages.

The Si-, GaAs-, InP-, or GaN-based photomixers and associated circuitrycan be fabricated by standard methods developed over the past decade.This includes interdigital electrodes defined in the photoconductive gapof the probe transmission line. The ultrafast Si-, GaAs-, InP-, orGaN-based active layers will either be implanted Si, epitaxiallow-temperature-grown (LTG) GaAs, epitaxial ErAs:GaAs, or some form ofInGaAs epitaxial film (lattice-matched to InP) developed by materialsscientists over the past few decades.

A second fabrication challenge exists in fiber-coupling of the diodelasers to the Tx and Rx photomixing probes 106, 108. The GaAs-basedphotomixers work best with 780-nm laser diodes, which are available ascommercial off-the-shelf (COTS) components. But 780-nm fiber is not aseasy to couple into and out-of single-mode fiber as the standard9-micron-core fiber so common in 1550-nm optical telecommunications. The˜6 micron core of 780-nm fiber is much more lossy and fragile than1550-nm fiber, so must be butt-coupled with special machines common tothe fiber-optic industry.

Referring to FIGS. 6-9, other manufacturers have been identified ashaving a vast experience in fiber-coupling at both 1550 and 780 nm;techniques of these manufacturers can be employed in fabricating probesof the subject innovation. Their bonding technique involves precisionfiber alignment using laser-radiation-induced photocurrent as thealignment metric. After the fiber is in place, a photo-active epoxy isapplied and then cured with UV light. They routinely perform fiberpackaging for a variety of fiber-coupled instruments that necessarilyhave to be field-deployable, so can survive manipulation, vibration,elevated temperature, and moisture.

One challenge created by the THz probe technology is theelectromagnetics. Dielectric rod antennas have never been scaled to beused at THz frequencies. And although they are fairly common atmicrowave frequencies, it is not clear whether the design rules thathave been developed apply to THz frequencies because these antennas aregenerally used for standard far-field transmit and receive functions,not the near-field coupling as required by our probe technology. Hence,research on the coupling included full-wave electromagnetic simulation.Electromagnetic simulation at THz frequencies using High FrequencyStructure Simulator (HFSS) is still probably the best option in thenumerical simulation industry. Research conducted in connection with thesubject innovation used HFSS-12 to model and design the photomixerprobes over a huge range of frequencies from roughly 100 GHz to 2.0 THz,and as expected, the coupling to the probe increases with frequency.

Once an optimal design is obtained in simulation, there is the requiredstep of verifying the performance experimentally. This is especiallyimportant at THz frequencies where various practical effects, such assurface roughness, can introduce extraneous losses that are nearlyimpossible to simulate realistically. A related challenge of anyhigh-frequency probe technology, dc- or ac-coupled, is calibration. Whatnearly every engineer desires from high-frequency probing of any deviceare the associated scattering (S) parameters. For two-port devices thesecan be obtained accurately only through a careful calibration procedure,the standard one utilizing “SOTL” standards (short, open, through,load). And such calibration is critical to the success of standardvector network analysis at all frequencies. Unfortunately, such SOTLstandards do not exist for the subject innovation, and it is not evenclear how to fabricate them. Therefore, an important aspect of theinnovation is the ability to carry out cross-calibration. The photomixerprobe technology can easily work down to 100 GHz or lower, therebyoverlapping the frequency range of existing VNAs, which operate up to˜500 GHz. Thus, the subject innovation can utilize a common, shared teststructure compatible with both conventional techniques and theinnovative probing techniques of the subject innovation. This can allowthe use of standard-VNA S parameters to quantify the photomixer probeperformance.

Given the anticipated levels of signal-to-noise ratio of the photomixer“microfork” probes (at least 50 dB) and comparable levels of dynamicrange, another possibility is characterization of prospective THzdevices by measurement of the noise spectrum. It is often true that thenoise spectrum from solid-state devices is governed by the same devicephysics and forward transfer function as the signal spectrum. In thecase of transistors, the output noise will be present even if theforward gain is very low, as required by thermodynamics. And at the highbias and current density levels that THz solid-state devices tend torequire, the shot-noise and thermal-noise effects are bound to be verystrong. Of course, this may require long integration times and tediousdata acquisition, but the noise spectrum could provide importantdiagnostic information about devices in which amplification is not evenpossible.

FIG. 10 is an illustration of the physical methodology of measuring orcharacterizing a solid-state device or circuit under test having afrequency up to 1.0 THz and beyond in accordance with aspects of theinnovation. At 1002, a Tx photomixing probe structure 106 and a Rxphotomixing probe structure 108 can be provided. At 1004, the Txphotomixing probe structure 106 creates an electromagnetic signal. At1006, the Tx photomixing probe structure 106 and the Rx photomixingprobe structure 108 can be ac-coupled to the solid-state device orcircuit under test 102 via an electromagnetic field, i.e. withoutelectrical contact between the probes 106, 108 and the planartransmission line 104. At 1008, the ac-coupling can occur by anelectromagnetic mode excited in the Tx photomixing probe structure andthe Rx photomixing probe structure, and can propagate throughpolarization currents. In each probe 1010, the polarization current canbe transformed to conduction current, or vice versa. At 1012, once thepolarization current is transformed to conduction current, theelectromagnetic field strength created by the Tx photomixing probestructure 106 can be measured by the Rx photomixing probe structure 108.

FIG. 11A illustrates a plan view of an alternative contact-free,ac-coupled probe structure 106, 108 in a balanced “orthorhombic” designconfiguration compatible with the CPW planar transmission line 104 inaccordance with aspects of the innovation (FIG. 12A is an illustrationof a corresponding probe structure 1200 that is an unbalancedorthorhombic design configuration, compatible with microstrip, etc). The“orthorhombic” probes 1100 and 1200 differ from the “microfork”structure of FIGS. 3 and 4 in that dielectric-waveguide structure of themicrofork is eliminated. Instead, the tapered transmission-line sectioncommon to all probes of the subject innovation can start at the bottomof the probe—the end closest to the transmission-line 104 under test. Inother words, the “orthorhombic” design includes the tapered,coplanar-waveguide region 1104 and an interdigital-electrode photomixer1106, but does not have the prongs of the “microfork” design shown inFIG. 3 or 4. The probe can be made from a high-resistivity Si, GaAs,InP, or GaN substrate material between 50 and 500 microns thick. Theoverall width of the probe 1100 can be between 10 and 100 micron,consistent with the size of planar transmission lines used in integratedcircuits at frequencies up to 1 THz and beyond.

The approximate dimensions illustrated in FIGS. 11B and 12B are based onpreliminary design considerations, including the width of the balancedor unbalanced transmission-line circuit in which the device-under-testis likely to be embedded and the area needed for fiber-optic coupling tothe photomixer element. The size of the gap distance shown in FIGS. 11Band 12B is again a tradeoff between strong coupling to the transmissionline under test and strong perturbation of the transmission line signal,both of which depend inversely on gap size. The rectilinear geometry ofthe “orthorhombic” probe complies with standard dicing and scribingtools, so it does not necessarily require the intricate lasermicromachining of the “microfork” described earlier. However, such lasermicromachining may be necessary for accuracy and surface-morphology(i.e., smoothness) reasons.

What has been described above includes examples of the innovation. Itis, of course, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing the subjectinnovation, but one of ordinary skill in the art may recognize that manyfurther combinations and permutations of the innovation are possible.Accordingly, the innovation is intended to embrace all such alterations,modifications and variations that fall within the spirit and scope ofthe appended claims. Furthermore, to the extent that the term “includes”is used in either the detailed description or the claims, such term isintended to be inclusive in a manner similar to the term “comprising” as“comprising” is interpreted when employed as a transitional word in aclaim. In addition, while the subject innovation uses examples with THzsignals, it is applicable to signals with other frequencies, especiallywithin the range of 0.5THz to 1.6THz.

What is claimed is:
 1. A device for measuring or characterizing asolid-state device or an integrated circuit at frequencies up to 1.0 THzand beyond comprising: a transmitting photomixing probe; and a receivingphotomixing probe; wherein the transmitting photomixing probe and thereceiving photomixing probe are ac-coupled to the solid-state device orthe integrated circuit.
 2. The device of claim 1, wherein eachphotomixing probe comprises a photomixer and a planar-transmission-linestructure.
 3. The device of claim 2, wherein each photomixing probefurther comprising an optical fiber coupling two lasers to thephotomixer.
 4. The device of claim 3, where in the two lasers have amutually coherent but tunable frequency difference.
 5. The device ofclaim 2, wherein each photomixing probe further comprises adielectric-waveguide structure.
 6. The device of claim 1, wherein eachphotomixing probe comprises a high resistivity semiconductor substrate.7. The device of claim 1, wherein the solid-state device or theintegrated circuit is embedded in a planar transmission line, andwherein an output from the solid-state device or integrated circuit issampled in the planar transmission line.
 8. The device of claim 7,wherein the distance between the planar transmission line and each ofthe photomixing probes is in the range of 1 and 100 micron, thethickness of each photomixing probe is in the range of 10 to 1000micron, and the height of each photomixing probe in the range of 0.1 to10 mm.
 9. The device of claim 7, wherein the coplanar transmission lineis a coplanar waveguide, a slot-line, a microstrip, or a twin line. 10.The device of claim 1, wherein the ac-coupling occurs via an interactionbetween a polarization current in at least one of the transmittingphotomixing probe or the receiving photomixing probe, and a fringingfield just above the solid-state device or the integrated circuit undertest.
 11. The device of claim 10, wherein the polarization current istransformed to a conduction current in the at least one of thetransmitting photomixing probe or the receiving photomixing probe, andwherein once the polarization current is transformed to the conductioncurrent, an electromagnetic field strength is measured by the receivingphotomixing probe structure.
 12. The device of claim 10, wherein theconduction current is transformed to the polarization current in the atleast one of the transmitting photomixing probe or the receivingphotomixing probe, and wherein once the conduction current istransformed to the polarization current, an electromagnetic fieldstrength is created by the transmitting photomixing probe.
 13. Thedevice of claim 1, wherein at least one of the transmitting photomixingprobe or the receiving photomixing probe comprises one or more“microforks.”
 14. The device of claim 1, wherein at least one of thetransmitting photomixing probe or the receiving photomixing probe has an“orthorhombic” configuration.
 15. The device of claim 1, wherein each ofthe transmitting photomixing probe and the receiving photomixing probecomprises: a tapered, coplanar-waveguide region; and aninterdigital-electrode region.
 16. The device of claim 1, wherein asignal generated by the transmitting photomixing probe is modulated viaone of an amplitude modulation or a frequency modulation.
 17. The deviceof claim 1, wherein at least one of the transmitting photomixing probeor the receiving photomixing probe comprises a substrate that comprisessilicon, gallium arsenide, gallium nitride, or indium phosphide.
 18. Thedevice of claim 1, wherein at least one of the transmitting photomixingprobe or the receiving photomixing probe was created at least in partvia a laser micromachining process.
 19. A method of measuring asolid-state device or an integrated circuit at an RF frequency up to 1.0THz and beyond comprising: providing a transmitting photomixing probestructure and a receiving photomixing probe structure; and coupling thetransmitting photomixing probe structure and the receiving photomixingprobe structure to the solid-state device or the integrated circuitwithout electrical contact.
 20. The method of claim 19, wherein thesolid-state device or the integrated circuit is embedded in a planartransmission line, and wherein an output from the solid-state device orintegrated circuit is sampled in the same planar transmission line. 21.The method of claim 19, further comprising modulating a signal from thetransmitting photomixing probe structure via one of an amplitudemodulation or a frequency modulation.